1. Field of the Invention
The present invention relates to a time constant detecting circuit and a time constant adjusting circuit. The time constant detecting circuit serves to detect variations of a time constant of a circuit including resistors and capacitors such as, for example, an RC active filter incorporated in an LSI, and the time constant adjusting circuit serves to adjust the variations of such a time constant.
2. Description of the Prior Art
A monolithic LSI includes therein a circuit which incorporates resistors and capacitors as constituent elements such as, for example, an RC active filter. This however causes a problem in that the element values become very large in their absolute values, as well known in prior practice. More specifically, variations of element values of resistors and capacitors are as small as 1% or less in the same chip but are as large as 20%.about.40%, among different chips.
Construction of such an active filter using resistors and capacitors therefore causes variations of a cutoff frequency ranging from 40% to 100% if variations of such element values range within +/-30%, and thus the cutoff frequency is widely varied and it is very unlikely to produce filters which have a desired accuracy.
To solve such problems of the prior art, recent practice of the construction of filters incorporated in monolithic LSIs are made in the form of switched-capacitor filter type circuits instead of RC type ones. Such switched capacitor filter type circuits, however, surely require using prevention filters and smoothing filters at a preceding stage and at a following stage because of a sampling type construction, which filters, must be constructed with RC active filters respectively. The aforementioned variations of cutoff frequency are thus inevitable and bring about several severe problems in designing a circuit.